Drilling fluids used in the drilling of subterranean oil and gas wells as well as other drilling fluid applications and drilling procedures are known. In rotary drilling there are a variety of functions and characteristics that are expected of drilling fluids, also known as drilling muds, or simply "muds". The drilling fluid is expected to carry cuttings up from beneath the bit, transport them up the annulus, and allow their separation at the surface while at the same time the rotary bit is cooled and cleaned. A drilling mud is also intended to reduce friction between the drill string and the sides of the hole while maintaining the stability of uncased sections of the borehole. The drilling fluid is formulated to prevent unwanted influxes of formation fluids from permeable rocks penetrated, and also often to form a thin, low permeability filter cake which temporarily seals pores, other openings and formations penetrated by the bit. The drilling fluid may also be used to collect and interpret information available from drill cuttings, cores and electrical logs. It will be appreciated that within the scope of the claimed invention herein, the term "drilling fluid" also encompasses "drill-in fluids" and "spotting fluids".
Spotting fluids are typically used to free stuck pipe during a drilling operation. Differential pressure sticking is the most common cause of stuck pipe in drilling operations, and circumstances which lead to differential sticking include overbalanced formation pressure and drill pipe that is stationary for a long amount of time. As drilling fluid is lost into the formation, the small pores of the formation face serve as filters, trapping the solids present in the mud system and causing a "filter cake" build up around the stationary pipe. Spotting fluids crack this filter cake and wet the space between the formation and the pipe, allowing the stuck pipe to be freed. Diesel and mineral oil-based fluids are conventionally used as spotting fluids, and are employed as a "slug" or "pill" within an aqueous drilling fluid.
Drilling fluids and such fluid systems are typically classified according to their base material. In water-based muds, solid particles are suspended in water or brine. Oil can be emulsified in the water, typically with the use of an emulsifier. Nonetheless, the water is the continuous phase. Brine-based drilling fluids, of course are a water-based mud in which the aqueous component is brine. Oil-based muds are the opposite. Solid particles are suspended in oil and water or brine is emulsified in the oil and therefore the oil is the continuous phase. Oil-based muds which are water-in-oil emulsions are also called invert emulsions. Such invert emulsions typically use an emulsifier, and brine is used as the discontinuous aqueous phase, obtained from a convenient source. Synthetic-based fluid systems are being developed to solve special challenges in hydrocarbon recovery. Such synthetic-based fluids include, but are not necessarily limited to, ester base fluids, such as ester oils (also sometimes called esterified oils) and isomerized-olefin based systems such as ISO-TEQ.RTM. from Baker Hughes INTEQ.
Polymers have been used in the past as viscosifiers in drilling fluid systems to carry or suspend cuttings, weighting agents and other solids in the fluids, such as invert emulsions. The use of new fluid systems, such as the synthetic systems, renders some conventional additives and agents ineffective or problematic.
It is apparent to those selecting or using a drilling fluid for oil and/or gas exploration that an essential component of a selected fluid is that it be formulated to achieve all of the necessary characteristics for the specific end application. Because the drilling fluids are called upon to do a number of tasks simultaneously, this desirable balance is not always easy to achieve.
It would be desirable if compositions and methods could be devised to aid and improve the ability of drilling fluids to simultaneously accomplish these tasks, particularly for the new synthetic drilling fluid systems.